ABSTRACT: Individuals of the marine snail Trichotropis cancellata can either suspension feed or steal food from tube-dwelling polychaete worm hosts (kleptoparasitism). Because kleptoparasitism is facultative in this case, the performance of both
members can be evaluated separately and together. I used field experiments to compare the growth of suspension feeding snails with that of snails (either singly or in groups) on Serpula columbiana polychaete hosts. All sizes of T.
cancellata grew more quickly (up to 18 times faster) when parasitizing S. columbiana than when restricted to suspension feeding. Kleptoparasitism provided a proportionally greater growth benefit for small and medium-sized snails than for larger
ones, suggesting that smaller gastropods are more food-limited when suspension feeding. Fecundity was positively correlated with snail size, so the growth benefits of kleptoparasitism are likely to be selectively advantageous. Increasing numbers of
medium-sized snails on a host reduced individual snail growth rates, suggesting competition for the worm¹s food. All snails were equally affected; there was not a superior competitor among medium-sized snails. Parasitism was experimentally demonstrated
because serpulid worms grew their tubes more slowly when supporting parasitic snails and were increasingly negatively affected by additional medium-sized parasitic snails. Adding large parasitic snails did not exacerbate the negative impact on worm tube
growth, so I concluded that a single large snail reduces worm hosts to their minimal rate of tube extension. Suspension feeding T. cancellata gain additional resources when they switch to kleptoparasitism.